1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a bistable chiral splay nematic (BCSN) mode liquid crystal display device being switchable between a memory mode and a dynamic mode.
2. Discussion of the Related Art
Recently, as the society has entered in earnest upon an information age, a field of display devices that represent all sorts of electrical signals as visual images has developed rapidly. Particularly, since the LCD device has characteristics of light weight, thinness and low power consumption, the LCD device has been widely used as a substitute for the cathode-ray tube display device.
A related art liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has high resolution and excellent characteristics for displaying moving images, the AM-LCD device has become widely used.
FIG. 1 is an exploded perspective view of a related art liquid crystal panel. As shown in FIG. 1, the liquid crystal panel includes an array substrate 10, a color filter substrate 20, and a liquid crystal layer 30. The array substrate 10 and the color filter substrate 20 face each other, and the liquid crystal layer 30 is interposed therebetween.
The array substrate 10 includes a first substrate 12, a gate line 14, a data line 16, a thin film transistor (TFT) “Tr”, and a pixel electrode 18. The gate and data lines 14 and 16 are formed on the first substrate 12 and cross each other to define a pixel region “P”. The TFT “Tr” is formed at a crossing portion of the gate and data lines 14 and 16. The pixel electrode 18 is formed in the pixel region “P” and connected to the TFT “Tr”.
The color filter substrate 20 includes a second substrate 22, a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 is formed on the second substrate 22 and has a lattice shape. The black matrix 25 corresponds to a non-display region of the first substrate 12. The non-display region of the first substrate 12 includes the gate and data lines 14 and 16 as well as the TFT “Tr”. The color filter layer 26 corresponds to the pixel region “P” and includes red, green and blue color filter patterns 26a, 26b and 26c. The common electrode 28 is formed on the black matrix 25 and the color filter layer 28. The common electrode 28 generates an electric field with the pixel electrode 18 such that the liquid crystal layer 30 is driven by the electric field.
Though not shown, a seal pattern is formed along edges of the first and second substrates 12 and 22. The seal pattern prevents the liquid crystal layer 30 from overflowing. In addition, first and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 and between the second substrate 22 and the liquid crystal layer 30. A polarization plate may be formed on an outer surface of one of the first and second substrates 12 and 22. A backlight assembly is formed on a rear side of the first substrate 12 to apply light into the liquid crystal panel. When a scan signal is applied to the TFT “Tr” through the gate line 14 to turn on the TFT “Tr”, an image signal is applied to the pixel electrode 18 through the data line 16 such that an electric field is generated between the pixel electrode 18 and the common electrode 28. As a result, the liquid crystal molecules in the liquid crystal layer 30 are driven by the electric field to display images.
The liquid crystal molecules may be classified into a nematic mode, a smectic mode and a cholesteric mode. Among these liquid crystal molecules, the nematic mode liquid crystal molecule, by which light is strongly scattered, is widely used.
An electro-optic effect of the liquid crystal molecules is to generate an electro-optical modulation by changing optical properties of a liquid crystal cell. The electro-optic effect is generated by changing an arrangement of the liquid crystal molecules.
Arrangement of the nematic mode liquid crystal molecules is continuously changed with an electric field. Generally, the nematic mode liquid crystal molecules are driven by an electric field in a twisted nematic (TN) mode or an in-plane switching (IPS) mode. The nematic liquid crystal molecules are aligned to be parallel to a substrate and twisted. In the TN mode LCD device, the nematic liquid crystal molecules are driven by a vertical electric field. In the IPS mode LCD device, the nematic liquid crystal molecules are driven by a horizontal electric field.
Recently, various display devices have been introduced to meet customers' requirements. Particularly, devices having advantages in a thickness, a weight and power consumption are introduced. In addition, the devices are required to have functions of displaying a moving image and an e-book or an e-paper. Accordingly, an LCD device being switchable in a dual mode, e.g., a dynamic mode for the moving image and a memory mode for the e-book or the e-paper is required. To meet the requirements, a bistable chiral splay nematic (BCSN) mode LCD device is introduced.
In the BCSN mode LCD device, the memory mode uses a bistable principle in a splay state and a π-twist state, and the dynamic mode uses a switching principle between a low bend state and a high bend state. Unfortunately, the memory mode and the dynamic mode in the BCSN mode LCD device have a difference in a black state, it is very difficult to provide a compensation film for compensating the black state in both the memory mode and the dynamic mode. Namely, because the black state in only one of the memory mode and the dynamic mode is optimized by the compensation film, the black state in the other one of the memory mode and the dynamic mode is deteriorated such that there is a disadvantage in a contrast ratio. As a result, there is a problem in an image display property of the LCD device.